The Space Launch System (SLS) is NASA’s new super heavy-lift launch vehicle designed to enable human exploration beyond low Earth orbit. As the most powerful rocket ever built by NASA, SLS will provide the capability to send astronauts, cargo, and scientific payloads to the Moon and potentially destinations like Mars. The SLS program involves an evolvable design with different block configurations that increase performance over time. This article examines the initial Block 1 configuration of SLS and the planned Block 1B upgrade, exploring their designs, capabilities, and roles in NASA’s Artemis program for lunar exploration.
SLS Block 1 Overview
The SLS Block 1 represents the initial configuration of the Space Launch System. It builds upon proven hardware and technologies from the Space Shuttle program while incorporating new developments to create an exploration-class heavy lift vehicle. Block 1 is optimized to launch the Orion spacecraft and send it on missions to lunar orbit.
Core Stage
At the heart of SLS Block 1 is the massive core stage. Standing at 212 feet tall with a diameter of 27.6 feet, the core stage contains the vehicle’s fuel tanks and primary propulsion system. It holds over 730,000 gallons of super-cooled liquid hydrogen and liquid oxygen propellants to feed the four RS-25 engines mounted at its base.
The core stage is manufactured at NASA’s Michoud Assembly Facility in New Orleans using state-of-the-art manufacturing equipment. It is constructed primarily of aluminum alloy and comprises five major sections – the forward skirt, liquid oxygen tank, intertank, liquid hydrogen tank, and engine section.
RS-25 Engines
SLS Block 1 utilizes four RS-25 engines to provide primary propulsion for the core stage. These engines previously served as the Space Shuttle main engines, accumulating an impressive flight history over 135 missions. For SLS, the RS-25 engines are being operated at a higher power level, producing 512,000 pounds of thrust each at 109% of rated power.
The RS-25 is a staged-combustion engine that operates at extreme temperatures and pressures. It burns liquid hydrogen and liquid oxygen propellants to generate tremendous thrust while achieving high efficiency. The engines can throttle between 65% to 109% thrust, allowing for precise control during the launch sequence.
Solid Rocket Boosters
Flanking the core stage are two five-segment solid rocket boosters that provide the majority of thrust at liftoff. These boosters are derived from the four-segment versions used on the Space Shuttle but with an additional propellant segment for increased performance. Each booster measures 177 feet tall and 12 feet in diameter.
The five-segment boosters generate a combined 7.2 million pounds of thrust at ignition, over 75% of the vehicle’s total liftoff thrust. They burn for approximately 126 seconds before separating from the core stage. The boosters utilize a polybutadiene acrylonitrile (PBAN) propellant and feature several upgrades over their Shuttle predecessors.
Upper Stage
SLS Block 1 employs the Interim Cryogenic Propulsion Stage (ICPS) as its upper stage. The ICPS is derived from the Delta IV rocket’s Delta Cryogenic Second Stage and is powered by a single RL10B-2 engine. It measures 45 feet long with a diameter of 16.7 feet.
The ICPS provides in-space propulsion to accelerate Orion toward its lunar trajectory after separation from the core stage. It carries 19,750 gallons of liquid hydrogen and 5,700 gallons of liquid oxygen. The RL10B-2 engine generates 24,750 pounds of thrust in vacuum and can restart multiple times in space.
Orion Stage Adapter
Connecting the ICPS to the Orion spacecraft is the Orion Stage Adapter (OSA). This conical structure provides the interface between the 16.7-foot diameter of the ICPS and the 18-foot diameter of Orion’s service module. On uncrewed test flights, the OSA can also accommodate small satellite payloads for deployment after Orion separation.
SLS Block 1 Performance
The SLS Block 1 configuration is capable of delivering over 95 metric tons (209,000 pounds) to low Earth orbit. For lunar missions, it can send more than 27 metric tons (59,500 pounds) on a trans-lunar injection trajectory. This includes the Orion spacecraft, which has a mass of approximately 57,000 pounds when fully fueled.
At liftoff, SLS Block 1 generates approximately 8.8 million pounds of thrust, about 15% more than the Saturn V rocket used for the Apollo missions. The vehicle stands 322 feet tall and has a gross liftoff mass of 5.75 million pounds.
SLS Block 1 Mission Profile
A typical SLS Block 1 lunar mission follows this general sequence:
- The two solid rocket boosters ignite at T-0, followed almost immediately by ignition of the four RS-25 engines.
- SLS lifts off and clears the launch tower, beginning its ascent.
- Around 2 minutes into flight, the solid rocket boosters burn out and separate from the core stage.
- The RS-25 engines continue firing, powered by propellants from the core stage tanks.
- At approximately 8 minutes after liftoff, the core stage engines cut off and the stage separates.
- The ICPS engine ignites to place the vehicle into a stable orbit.
- After reaching orbit, the ICPS reignites to perform the trans-lunar injection burn, sending Orion toward the Moon.
- The ICPS separates and Orion continues its journey using its own propulsion system.
This mission profile allows SLS Block 1 to deliver Orion to a lunar flyby trajectory or into orbit around the Moon, depending on mission requirements.
SLS Block 1B Overview
SLS Block 1B represents the first major upgrade to the Space Launch System’s capabilities. It replaces the Interim Cryogenic Propulsion Stage with the larger and more powerful Exploration Upper Stage (EUS). This change enables SLS to send both crew and large cargos to the Moon in a single launch.
Exploration Upper Stage
The defining feature of SLS Block 1B is the new Exploration Upper Stage. The EUS is significantly larger than the ICPS, measuring 45 feet long and 27.6 feet in diameter – the same diameter as the core stage. It is powered by four RL10C-3 engines, each generating 24,750 pounds of thrust in vacuum.
The EUS carries about 285,000 pounds of liquid hydrogen and liquid oxygen propellants. Its four engines can restart multiple times in space, providing flexible maneuvering capabilities for various mission profiles. The stage’s large propellant capacity allows it to deliver heavy payloads to a range of destinations beyond low Earth orbit.
Universal Stage Adapter
To accommodate larger payloads, SLS Block 1B introduces the Universal Stage Adapter (USA). This conical structure connects the EUS to either the Orion spacecraft or a payload fairing. The USA measures about 32.8 feet tall and varies in diameter from 27.6 feet at its base to 18 feet at the top.
For crewed missions, the USA provides a large enclosed volume between the EUS and Orion that can house additional cargo or habitation modules. On dedicated cargo flights, the USA connects the EUS to a large payload fairing to encapsulate satellites or other exploration systems.
Payload Accommodations
SLS Block 1B offers significantly expanded payload capabilities compared to Block 1. In its crew configuration, it can carry both the Orion spacecraft and an additional co-manifested payload up to 10 metric tons. This allows delivery of habitat modules, landers, or other large components to lunar orbit along with a crew.
In its cargo configuration, SLS Block 1B can use payload fairings up to 8.4 meters (27.6 feet) in diameter and 27.4 meters (90 feet) tall. This provides a large enclosed volume for launching satellites, exploration systems, or planetary probes.
SLS Block 1B Performance
The enhanced upper stage of SLS Block 1B provides a major boost to the vehicle’s performance. In its crew configuration, Block 1B can deliver 34-37 metric tons to trans-lunar injection. For missions to low lunar orbit, it can carry up to 42 metric tons including Orion.
The cargo configuration of SLS Block 1B, without Orion, can send up to 46 metric tons on a trans-lunar injection trajectory. To low Earth orbit, it can launch over 105 metric tons. This represents a significant increase over Block 1 capabilities.
At liftoff, SLS Block 1B generates the same 8.8 million pounds of thrust as Block 1, provided by the core stage engines and solid rocket boosters. The vehicle stands 364 feet tall in its crew configuration.
SLS Block 1B Mission Flexibility
The improved performance of SLS Block 1B enables more ambitious missions beyond low Earth orbit. Some potential mission types include:
- Delivering both crew and large cargo to lunar orbit in a single launch
- Sending heavy robotic science missions to the outer solar system
- Launching fully-fueled landers or habitat modules to lunar orbit
- Orbiting large space telescopes beyond Earth orbit
- Boosting planetary defense payloads on fast trajectories
The ability to co-manifest large payloads with crew allows for more efficient assembly of systems in lunar orbit. This is expected to play an important role in establishing sustainable lunar exploration capabilities.
Commonalities Between Block 1 and Block 1B
While the upper stage represents a major change, SLS Block 1B retains several core elements from the Block 1 design:
- The same core stage with four RS-25 engines
- Twin five-segment solid rocket boosters
- Overall vehicle diameter of 8.4 meters (27.6 feet)
- Similar liftoff thrust and initial ascent profile
- Use of the Orion spacecraft for crewed missions
This commonality helps streamline manufacturing and operations as the SLS program evolves. The shared core stage and booster design allows for a flexible production and launch cadence to meet varying mission needs.
Development Status
SLS Block 1 completed its first test flight in November 2022 with the uncrewed Artemis I mission, successfully launching the Orion spacecraft on a flight around the Moon. This validated the overall vehicle design and performance.
Production of core stages and other elements for future Block 1 flights is ongoing. The next mission, Artemis II, will be the first crewed flight of SLS and Orion, sending astronauts around the Moon. This is targeted for late 2024 or 2025.
Development of the Exploration Upper Stage for Block 1B is underway, with critical design review completed in 2020. As of 2023, the first flight of the SLS Block 1B configuration is planned for the Artemis IV mission, currently targeted for 2028. This mission would deliver crew and the first module of the lunar Gateway station to orbit around the Moon.
Challenges and Considerations
The SLS program faces several challenges as it moves forward with Block 1 flights and Block 1B development:
Cost and Sustainability
As a large, expendable rocket, each SLS launch carries a high cost – estimated at over $2 billion per flight for the initial missions. NASA is working to reduce per-flight costs as the program matures, but SLS will remain an expensive system to operate. This raises questions about long-term sustainability and flight rates.
Production Capacity
Current facilities can produce about one SLS core stage per year. Increasing this production rate to support more frequent launches presents logistical and manufacturing challenges. Balancing production capacity with mission demand and budget constraints requires careful planning.
Evolving Space Industry
The emergence of commercial heavy-lift rockets like SpaceX’s Falcon Heavy and Starship creates a more competitive launch market. While SLS offers unique capabilities, NASA must justify its continued development and use in light of potentially lower-cost alternatives for some missions.
Technical Risks
Developing the Exploration Upper Stage and other Block 1B elements involves technical challenges and risks. Any significant delays could impact the timeline for upgrading SLS capabilities and supporting more ambitious lunar missions.
Launch Infrastructure
SLS requires extensive ground systems and launch infrastructure. Transitioning from Block 1 to Block 1B operations at Kennedy Space Center will involve facility modifications and new procedures to be validated.
Future Evolution
Beyond Block 1B, NASA has studied concepts for further evolution of the SLS design. A potential Block 2 configuration would introduce advanced boosters to replace the five-segment solid rockets, potentially increasing payload capacity to 130 metric tons to low Earth orbit. However, plans for Block 2 remain preliminary, with focus currently on executing Block 1 flights and completing Block 1B development.
Role in NASA’s Artemis Program
SLS Block 1 and Block 1B form a central part of NASA’s Artemis program to return humans to the Moon and establish sustainable lunar exploration. The rockets will launch crews aboard Orion to lunar orbit and deliver components for the planned Gateway station.
Initial Block 1 missions focus on demonstrating capabilities and proving out systems for lunar return. As Block 1B becomes available, it will enable launching co-manifested payloads like landers and habitats along with crew. This supports a buildup of infrastructure and capabilities in lunar orbit and on the surface.
The high performance of SLS allows for flexibility in mission design, potentially enabling faster transit times to the Moon or launching fully-fueled landers. As the Artemis program progresses, SLS is expected to play an ongoing role in crew transport and delivering large cargos to lunar orbit.
Summary
The Space Launch System represents NASA’s new generation of super heavy-lift launch capability for human exploration beyond Earth orbit. The initial Block 1 configuration provides the performance needed to send Orion missions to the Moon, building on proven technologies to enable a rapid return to lunar exploration.
The planned upgrade to SLS Block 1B with the Exploration Upper Stage will mark a significant expansion of capabilities. By enabling co-manifested payloads with crew or large dedicated cargo launches, Block 1B supports more ambitious missions and infrastructure development in deep space.
As the SLS program moves forward, it faces challenges related to costs, production rates, and evolving capabilities in the commercial launch sector. However, its unique heavy-lift capacity and ability to launch both crew and large payloads continues to make it a cornerstone of NASA’s plans for human exploration of the Moon and eventually Mars.
The Space Launch System’s evolution from Block 1 to Block 1B demonstrates NASA’s phased approach to expanding deep space capabilities. This strategy allows for early lunar missions while developing more advanced systems to support long-term exploration goals. As flights of both configurations progress in the coming years, SLS will play a central role in humanity’s return to the Moon and journeys beyond.
